I'd also add that I am an amateur long-distance runner and just completed a medium distance run after fasting all day and performed fine. Had it been a race, probably less so. But the myths of caloric necessities have been greatly exaggerated.
As an personal anecdote, I enjoy climbing in the mornings before having food, that's a few good hours of intermittent sprint type activity.
I have to head out running before 06:00 ( ex-farm border collie dog still has early-rising demands ) so need to eat about 05:30. On a couple of occasions I overslept and skipped breakfast. It was terrifying, I barely had enough energy to make it back home and was shaking. All my body wanted when I returned was sugar, now.
So in my case at least a bowl of museli / milk / yoghurt and a banana is the minimum possible breakfast.